Point defects in monolayer WSi₂N₄ and MoSi₂N₄

Point defects critically govern the properties of two-dimensional semiconductors, yet their atomic-scale characteristics in the emerging MoSi₂N₄ family remain unexplored. Here we combine atomic-resolution scanning transmission electron microscopy and first-principles calculations to systematically investigate point defects in monolayer semiconducting WSi₂N₄ and MoSi₂N₄. We identify ten distinct defect types, with Si-for-top-N antisites (Si_N(t)), double-middle-N divacancies (V_N(m)2), and double-top-N divacancies (V_N(t)2) being the most abundant. These defects induce mobility modulation and bandgap reduction - in some cases leading to complete band closure and insulator-to-metal transitions - and, in certain configurations, give rise to spin-polarized bands with localized magnetic moments. Additionally, we observe low-dimensional assemblies formed via defect self-organization, including 2D Si_N(t) networks and 1D Si2_Mo chains. Our findings establish fundamental defect-property relationships and provide insights for defect-driven engineering of electronic and magnetic states in 2D WSi₂N₄ and MoSi₂N₄ semiconductors.